The invention is particularly, but not exclusively, applicable to low-power dc-to-dc converters of the kind used as computer power supplies, for example. In such converters the dc input is switched across the primary of an isolation transformer, and the secondary voltage is filtered and rectified to provide the output dc. It is well known that the weight and bulk of the transformer, and its cost, can be reduced by increasing the switching frequency but only at the expense of increasing the sophistication of the associated circuitry.
For example, it is known to provide a 12 V-5 V converter with a power handling capacity of a few watts operating in the range of 1.5-2.0 MHz, controlled by pulse width modulation of the primary switching. At these frequencies, the topography of the magnetic parts and associated circuits becomes important, and additional features must be introduced to control ripple currents and electromagnetic interference.
In certain known designs these requirements lead to the use of an isolation transformer, a second transformer used as an output filter and feedback signal source, and further inductors to inhibit ripple and common mode currents. These have normally been provided as discrete, spaced components leading to relatively high space requirements and costs. It has been proposed recently to use integrated magnetics where multiple functions are performed on a single magnetic structure. However, these proposals are based on arrangements where the various circuits are coupled with each other with the windings interlinked, and require specially made magnetic structures which are difficult to produce in quantity, typically relying on E-cores in which one leg has an air gap and the others have different size pole pieces.